In recent years, interest is growing in a transcranial magnetic stimulation therapy as a treatment method for many patients with nervous diseases in which drug treatment is not always effective. The transcranial magnetic stimulation therapy is a relatively new treatment method for providing treatment and/or relieving a symptom by applying magnetic stimulation to a specific region (for example, an intracerebral nerve) of a brain with a magnetic field generator disposed on a scalp surface of the patient. Unlike the conventional electric stimulation method which requires craniotomy and an indwelling electrode to which a patient feels a strong resistance, the transcranial magnetic stimulation therapy is expected to become widespread as the non-invasive treatment method with a less burden on the patient.
In a specific technique of the transcranial magnetic stimulation therapy, passage of current through a coil located near the scalp surface of the patient locally generates a minute pulse magnetic field to cause eddy current in a cranium by using the principle of electromagnetic induction, thereby providing stimulation to the intracerebral nerve immediately below the coil (for example, see Patent Document 1).
According to Patent Document 1, a refractory neuropathic pain is effectively reduced by the transcranial magnetic stimulation treatment performed by the above method and a high pain reducing effect is obtained by precise local stimulation. However, it is also revealed that an optimum stimulation region slightly differs depending on an individual patient.
Accordingly, in order to obtain the high effect of the transcranial magnetic stimulation therapy, it is necessary to specify, for each patient, the optimum stimulation region of a patient's head, namely, it is necessary to correctly perform three-dimensional positioning of the treatment coil with respect to the patient's head. It is also known that the obtained effect may differ depending on an orientation (posture) of the treatment coil even if the treatment coil is located at an identical position.
As to the positioning of the treatment coil, it is known that the treatment coil is positioned with respect to the patient's head with an optical tracking system in which an infrared beam is used (for example, see Patent Documents 2 and 3). Some systems are already commercially available and used in a clinical application.
As described above, in order to obtain the pain reducing effect by the transcranial magnetic stimulation therapy, it is necessary to specify the optimum stimulation region of the patient's head to correctly provide the stimulus to the intracerebral nerve. It is difficult to know the correct position of the brain existing in the cranium from the outside. However, the position of the brain can correctly be know using three-dimensional information about a head MRI image (Magnetic Resonance Imaging). While referring to the cranial three-dimensional information obtained by the MRI image, an operator (such as a doctor) of the transcranial magnetic stimulation therapy can guide the treatment coil to the optimum stimulation region of the patient's head to correctly provide the magnetic stimulation using a positioning function of the optical tracking system.
Conventionally, in the case where the optical tracking system is used in the transcranial magnetic stimulation therapy, infrared reflective markers are installed in a fixed position associated with the patient's head (for example, a bed on which the patient lies) and the treatment coil, a current position of the treatment coil is estimated from a positional relationship between the two obtained by detection of the markers, and the treatment coil is guided to the optimum stimulation region of the patient's head while the cranial three-dimensional information obtained by the MRI image is referred to. Accordingly, the correct positioning is required between the patient's head and the MRI image. An eye, an ear, a nose, or the like is designated using a calibration marker while the patient's head is fixed to the bed, whereby the patient's head is correctly positioned with respect to the MRI image.